Subterranean operations are typically performed to retrieve hydrocarbons from subterranean formations. An example of such subterranean operations is a drilling operation. When performing drilling operations in a subterranean formation, segments of pipe called casing may be joined and placed in a wellbore drilled to a first intermediate depth to form a casing string. Cement may then be pumped between the casing string and the wall of the wellbore to mechanically hold the casing in place and prevent fluid flow in an annulus formed between the inner wall of the wellbore and outer wall of the casing. In certain applications, it may be desirable to drill the wellbore further into the formation through the casing once the casing has been installed. Before drilling begins through the casing, one or more blowout preventers may be attached to the top of the casing. The blowout preventer may seal the top of the casing should excess pressure be encountered when drilling the wellbore deeper.
Blowout preventers are a type of valve often referred to in the oil and gas industry as “BOPs”. BOPs may be used to prevent blowouts during the drilling and production of hydrocarbon wells. A BOP may be installed at the wellhead to prevent the escape of pressure in an annular space between the casing and drill pipe, or in an open hole during drilling and completion operations. During the drilling operation from a drilling platform, the BOP may be located some distance below the drilling rig floor. The drilling platform may include a top drive which is suspended in the drilling derrick structure or a rotary table which is mounted within a circular opening in the floor. The top drive or rotary table is used to turn the drill string and support the drilling string assembly. The BOP may be mounted on top of the casing through which the drill string passes. Typical BOPs are massive structures, often weighing in excess of 300 tons in some deepwater drilling operations, and extending up to 50 feet from the top of the casing.
A capping stack may be used as a secondary closure device in the event of a blowout when the BOP has failed. Capping stacks are typically needed at drilling sites during emergencies (i.e., during a well blowout); therefore, transport and assembly time is of the essence. Capping stacks are currently typically transported to a drilling site using special aircraft (e.g., an Antonov aircraft) that are suited for carrying large components associated with a BOP. However, there are only a limited number of these specialized aircraft available worldwide. Additionally, there are limited areas to land these specialized aircraft. Further, each capping stack component typically has to be customized before each deployment to suit the aircraft. These factors extend the time delays associated with transport and assembly of capping stacks to drilling sites.
Typically, large components of a capping stack are transported individually to the drilling site and erected and pressure tested onboard the drilling platform. The erection and assembly of the capping stack components is referred to herein as “stacking” This stacking operation usually requires laborers to work in very close contact with these large components within a limited space onboard the drilling platform, often as the components are suspended overhead. Additionally, with the limited working height and space onboard the drilling platform, there are few lifting devices that can fit into this space to assist in this operation. Once the capping stack is in place, the equipment must be pressure tested to check its ability to perform properly. If any one of the components fails the pressure testing, the assembly process may have to be repeated to repair or replace the faulty component. This enhances the risks to personnel and the time required to bring the drilling rig to an operational state. Injuries ranging from incidental to serious have been experienced industry-wide due to this operation.
During the various “stacking” and component installation processes, alignment of components is also a critical factor. Virtually all capping stack components are typically assembled with the use of flanges. Consequently, proper alignment must exist between the mating pieces to be able to install gaskets, seals and the bolting required for holding the components together. This requirement is not trivial since the lack of ability to maneuver heavy flanges, one relative to the other, greatly increases the difficulty of installation in a safe and proper manner.
Another concern affecting the capping stacking operation is the lack of preventive maintenance performed on the lifting equipment. The lack of preventive maintenance can lead to the risk of failure of these devices. Since most of these lifting devices reside high off the ground and out of reach, regular maintenance is difficult to perform. This scenario is especially dangerous for the most common lifting devices where wire ropes or chains are the primary lifting means. Left unattended, these components can become prone to failure due to exposure to the inherently corrosive environment in and around the wellbore area.
Since the capping stack is an important piece of safety equipment involved in drilling operations, its functionality is essential. Quick assembly and transport of capping stack components is critical to wellbore operations given that capping stacks are only needed during emergencies. There is a need for a capping stack that is easier and faster to transport to well sites and assemble for use. A capping stack that is transportable on cargo aircraft and is able to be pressure tested at a wellbore location is desirable.